Pump jacks are used to pump oil and fluids out of wells across the world. Pump jacks are powered by electric or fuel-powered engines connected to a transmission using belts or sheaves. The transmission turns a crank that rocks a horsehead back and forth. The rocking horsehead reciprocates a polished rod connected by sucker rods to a downhole pump. Oil is returned up the well tubing and gas returns through the annulus between the tubing and casing. Vertical stroke pumping systems like the Rotaflex™ system have been developed as a replacement for pump jacks.